In response to a shift to use multiple values for transmission signals, a discrete multi-tone (DMT) scheme has been proposed as a transmission scheme to add a subcarrier modulation signal to a carrier light beam and thereby optically transmit data. Under the DMT scheme, for respective subcarriers whose frequencies are different from each other, negotiation is executed to optimize the transmission capacity according to frequency characteristics including those of the optical transmitter, the optical receiver, and the line path. In this negotiation, for example, bit/power mapping is executed for each of the subcarriers and the degree of multiple values of the modulated signal to be allocated to each of the subcarriers is accordingly varied. According to the DMT scheme, this scheme is thereby applicable also to a low-cost optical device whose band cannot be expanded to a high frequency, and enables an increase of the transmission capacity (for example, refer to Toshiki Tanaka, et al, “50 Gbps Class Transmission in Single Mode Fiber using Discrete Multi-Tone Modulation with 10G Directory Modulated Laser”, March 2012, OFC 2012, OSA; and Toshiki Tanaka, et al, “Discrete Multi-Tone Technology for 100G Ethernet (100 GbE)”, September 2012, IEEE 802.3 Geneva interim meeting).
A transmitting apparatus based on the DMT scheme has an optical transmitter and an optical receiver that are directly connected to each other (point-to-point connection). The transmitting apparatus allocates a subcarrier that is arbitrarily set from plural subcarriers to a communication link and, after securing the communication link between the transmitter and the receiver, executes the negotiation (for example, refer to David Lewis, et al, “400GE DMT Multi-Vendor Interoperability Requirements”, January 2015, JDSU).
A technique has been disclosed to determine a pilot signal and a control channel based on the reception quality of the pilot signal and that of each component carrier in multi-carrier radio communication (for example, refer to Japanese Laid-Open Patent Publication No. 2005-109743 and International Patent Application No. 2013-524672).